A gripping device of the described type is typically found in a denesting apparatus for denesting and discharging the lower tray or carton of a stack of nested product trays or cartons, for example associated to an egg packaging line.
FR 2320882 discloses a denesting apparatus for denesting dimpled trays. The denesting apparatus comprises two gripping units. The gripping units are mounted on a cross member that in its center is mounted to a piston rod of a central cylinder. Each gripping unit has at least three pivotable fingers and an intermediate cylinder. The fingers have a prick pin at their ends. The fingers of one gripping unit are operatively connected to the associated intermediate cylinder by a sliding plate. The fingers of one unit enclose a common dimple and, upon actuation of the associated intermediate cylinder, move radially inward over the same distance to grip the dimple. This known device has as for a disadvantage that the position of the dimples of the trays and the position of the gripping unit have to match quite accurately to assure a proper operation. If the gripping units and the dimples are mutually offset, the fingers will bump into the dimples and the fingers will not be able to enclose and grip the dimple in a correct way.
EP 0 041 304 A1 discloses a denesting apparatus for separating the lowest egg tray of a stack of egg trays. The denesting apparatus includes a gripping unit that is moveable up and down. The gripping unit comprises a bar provided with multiple gripping members that engage different dimples of the tray. The respective bars are linked to lever arms that are each pivotable about a shaft. The gripper members comprise prick pins for gripping into dimples of the egg tray. The mentioned pivot shafts are parallel and the prick pins are pointing in the opposite direction, whereby the opposing gripper members are each operating in a mutual opposite direction. The gripper members are connected to a mutually common actuator by means of links and a cam following system. The gripper members will thus all make the same stroke.
Although the denesting apparatus of EP 0 041 304 A1 already mitigates the problem of the denesting apparatus of FR 2320882 relating to the possible offset of the position of the dimples relative to the position of the grippers, it still is not satisfying.
Trays for eggs are often made of papier-maché (molded fiber) and their form may be somewhat irregular. Furthermore this type of trays is susceptible to changing dimensions over time, for example due to the presence of moist or dry air. Dimple-structure trays or boxes may also be made of plastics material and may have changing dimensions due to the application of heat for example during washing. The gripping members that are attached to the same common bar will move towards the respective dimples of the tray, but due to the rough and irregular shape of the tray and the dimples, not all gripping members may engage a dimple. When not all gripping members engage the dimples of the tray, the tray me be torn, and/or the tray may not be properly separated from the stack.
The invention has for an object to obviate this problem.